Munroe follows the international standards of the scientific community by primarily using the modern metric International System of Units, from the French Système international d’unités (SI). This coherent system has seven base units to measure different quantities and numerous derived units and prefixes. The seven SI base units are second for time; meter for length; kilogram for mass; ampere for electric current; kelvin for thermodynamic temperature; mole for amount of substance; and candela for luminous intensity. As an American writer, Munroe also uses some Imperial units such as pounds and stones to measure weight and uses miles and inches to measure distance. Using a combination of different units depending on the amounts measured and the context of the question allows him to choose the most apt system of measurement on a case-by-case basis and thus more effectively and clearly convey scale and size to his audience.

A branch of physics, thermodynamics deals primarily with heat, entropy, and energy. The name of the field was coined in the 19th century from the ancient Greek prefix “thermos,” meaning “warm/hot,” and the ancient Greek word dunamikos via the French dynamique, meaning “powerful,” to refer to a general theory of the relationship between mechanical energy and heat.